Pulse shifted array

ABSTRACT

A laser-radar receiver comprising an array of optical fibres, wherein the opposite ends of the optical fibres are connected to at least one electromagnetic radiation detector, each of the optical fibres having differing physical characteristics which result in known delays in the transmission time of pulsed electromagnetic radiation.

[0001] This invention relates to the field of laser-radar imaging andrelated sensor technology.

[0002] Conventional active laser-radar imaging systems provide an arrayof sensor elements which combine optically to capture an image of a bodyor target. The use of an active or pulsed light source such as a laserto illuminate the body or target generally provides an improved opticalreturn, thereby allowing a three dimensional image of a scene to becaptured. Such images comprise information relating to azimuth,elevation and range.

[0003] Where a laser is used as the active or pulsed light source toproduce the required optical returns, it is advantageous to utiliseshort duration laser pulses in order to help reduce the energy levelsrequired by the sensor systems. The use of such lasers helps achieve agreater range resolution.

[0004] Conventional sensor array detector elements used with active orpulsed light sources often have relatively long time constants whichrequire shorter duration pulses to be integrated into longer pulses.This can lead to a reduction in the range resolution of the system as awhole. The use of active pulse light sources as a means of achievinggreater range resolution has been made possible by the use ofQ-switching, whereby laser sources can achieve nanosecond pulsedurations. To help overcome the integration problems associated withconventional array detector elements, Avalanche Photo-Diodes (APD's)have been used. APDs can readily perform the required optical detectionand processing of short duration pulses, but problems still exist inrelation to the fabrication of APDs into arrays.

[0005] In a paper number #3065-04 presented at the SPIE AeroSensemeeting (Apr. 20-25, 1997, Orlando, Fla.), a prototype active imaginglaser-radar receiver was presented. The receiver incorporated an arrayof fibre coupled multi-channel receivers enables it to acquire imagesfrom a single laser pulse. Conventional scanned laser-radar imagingreceivers require multiple pulses to assemble full images and sufferfrom jitter and image tearing caused by platform or target instabilityand other environmental effects. The paper proposed the use of a singlepulse approach thereby eliminating distortions and providing highquality, high speed range based images.

[0006] The receiver as presented in the paper consisted of a focal planearray, formed of end polished multi-mode fibres. Each fibre acts as alight bucket, thereby capturing optical signals and relaying saidsignals to a series of detector elements. An array of APDs (APDs) wasthen utilised to detect and process light captured by the pixels formedby each end polished fibre-optic.

[0007] The configuration of the imaging laser-radar receiver aspresented in the above referenced paper, requires that each pixel in thefibre-optic array has an associated APD detector. It therefore followsthat, for example, a 24×24 array of pixel elements would require a totalof 576 APD detector and processing elements. This makes any such areceiver comparatively large, and expensive in terms of the number ofAPDs and the associated electronics. In addition to the physical sizeand cost of developing such a system, the APD detection and processingelectronics will remain largely dormant when a typical pulse repetitionrate of 1 kHz is used. This follows because the detector is required torespond to pulses of a few nanoseconds duration, thereafter lyingdormant for the remainder of the one millisecond pulse duration.

[0008] The invention provides for an imaging laser-radar receiver whichrequires substantially fewer detectors (and associated processingelectronics) by utilising fibre-optic delay lines to supply time shiftedpulses into each detector. The reduction in the number of detectors canprovide for a physically smaller and more compact receiver system alongwith a corresponding reduction in the costs associated with the numberof APDs required. Additionally, the invention provides flexibility inrelation to the physical location of both detectors and associatedelectronics, thereby providing for further benefits in terms ofpackaging volume and the use of otherwise redundant space in hostcontainers and vehicles.

[0009] Accordingly there is provided a laser-radar receiver comprisingan array comprised of the first ends of a plurality of optical fibres,wherein the corresponding opposite ends of said optical fibres areconnected to at least one electromagnetic radiation detector means, eachof said optical fibres having differing physical characteristics whichresult in known delays in the transmission time of plusedelectromagnetic radiation incident on said first ends of said opticalfibres to said at least one electromagnetic detector means.

[0010] The invention will now be described by way of example only withreference to the following drawings in which;

[0011]FIG. 1 shows a diagrammatic representation of a state of the artimaging laser-radar receiver.

[0012]FIG. 2 shows a diagrammatic representation of an imaginglaser-radar receiver in accordance with a first embodiment of theinvention.

[0013]FIG. 3 shows a diagrammatic representation of an imaginglaser-radar receiver which is a variant of that shown in FIG. 2.

[0014]FIGS. 4a and 4 b show a pulse train received by the apparatus ofFIGS. 2 and 3 respectively.

[0015]FIG. 5 shows an imaging laser-radar receiver in accordance with asecond embodiment of the invention.

[0016]FIG. 1 shows an array of nine fibre-optic cable end faces (pixels)4 each having a fibre-optic cable transmission path 6 for carryingoptical signals to a corresponding array of APD's 8. When a light source1 is incident on the array of pixels 2, each fibre acts as a lightbucket, capturing the optical signal 1 and relaying it via thefibre-optic transmission line 6 to the dedicated APD 8 for each cable.Each APD 8 provides the means for optical detection and processing ofthe light source 1, each having a corresponding output transmission line9 for supplying the light information on to a further processing means10 via input terminals 12. The further processing means 10 is thenutilised to construct a three-dimensional image of the body illuminatedby the light source. Each of the fibre-optic transmission lines 6 are ofsubstantially equal length, thereby providing all light sourceinformation 1 falling on any of the pixels 4 in phase to the APD's 8.

[0017]FIG. 2 shows an imaging laser-radar receiver in accordance withthe invention having an identical number of pixels 4 to that describedin the example shown in FIG. 1, but distinguished therefrom in that eachof said fibre-optic transmission cables 14 carry light sourceinformation 1 to a single APD 18. The invention utilises delays 16 inthe fibre-optic transmission lines 14 to provide time shifted pulses tothe APD 18. In the 3×3 “cluster” of pixels shown in FIG. 2, all ninepixels feed one APD 18. The centre pixel 24 has the shortest fibre-optictransmission cable 26 for transmitting light source information 1 to theAPD 18. Each of the remaining surrounding pixels are connected to theAPD 18 by fibre-optic transmission lines 14, each having correspondingdelays 16, each delay (d1, d2, d3 . . . d8) being different and eachdelay being provided in this example by a different length of fibre.This arrangement provides for the light source data 1 from the ninepixels to be multiplexed into the APD 18.

[0018] The selection of the centre pixel as that being the pixel havingthe shortest transmission path to the APD is for purposes of exampleonly and is not intended to represent a limiting feature of theinvention. Accordingly in an array, any one pixel could equally beselected to be that with the shortest time transmission path.

[0019] When comparing the system described in FIG. 1 with that of FIG.2, it is apparent that in the 3×3 example in FIG. 2 provides for anarrangement which requires eight less APDs in order to provide the samelight source data 1 to an APD for onward processing by the processorunit 20. This principle can be scaled to suit various arrays of pixelsretaining the same benefits. For example in a 5×5 array it would bepossible to utilise one centre pixel surrounded by twenty four furtherpixels, each utilising delays 16 in their respective opticaltransmission lines. When compared with a state of the art 5×5 arraysystem following the teaching described in FIG. 1, a comparable systemin accordance with the invention as described by FIG. 2 would use twentyfour fewer APD elements.

[0020] An example of typical values used to explain the invention shownat in FIG. 2 now follows. In a 3×3 array it will be assumed that thedelay lines 16 have lengths which are integer values (e.g. 40 m, 80 m,120 m, 160 m . . . ). Light source information 1 travelling through a 40m fibre having a refractive index of 1.5 will take 200 ns to travel fromthe pixel to the APD 18. The first pulse from the array will be from thecentre pixel, unless an adjacent pixel is imaging an object which ismore than 30 m (i.e. 40×1.5÷2) closer than the master pixel. Thiscondition is known as a “range ambiguity” and is a consequence of usingthe pulse shifted array approach of the invention. In the unlikely eventthat such an ambiguity occurs, the processing unit 20 would detect notone, but two pulses in a 200 ns period thereby indicating that a rangeambiguity problem or “false alarm” is present.

[0021] Another type of range ambiguity occurs when not all of the pixelsin the array capture an optical signal. This may occur for example, whenthe array is directed towards the edge of the body or target and not allof the pixels receive a reflected pulse.

[0022] Assuming that no range ambiguities exist in the example, the APD18 will see the pulse from the centre pixel followed by eight sequentialpulse from the surrounding pixels. Each pulse will occur at or about 200ns intervals depending on the physical relationship between the plane ofthe array of pixels and the angle to the light source. Hence, the trainof nine pulses will have been detected within about 1.6 μs (i.e. 8×200ns). Assuming a typical imaging at frame rate of 1 kHz it will beevident that many more surrounding pixel pulses could be detected in theexample given above, where a 1 kHz repetition rate implies a window of 1millisecond (1000 μs). The range to an object, regardless of whether itis the centre or a surrounding pixel can therefore be determined foreach pixel and a 3 dimensional image of the illuminated objectconstructed.

[0023] A further aspect to be taken into account when considering theexample in FIG. 2, is the total length of fibre-optic cable required. Inthe example the first surrounding pixel has an associated 40 m fibre,and the last (i.e. 8th) has a 320 m fibre. The length of fibre requiredfor each 3×3 array using the invention as described in accordance withFIG. 2 is 1.44 km. If this figure is now applied to a scaled up andrepresentative array size of 24×24 pixels, then utilising one APD pernine pixels results in a requirement for sixty four 3×3 arrangements.Accordingly, the total length of fibre-optic cable required for a 24×24pixel array would equal 92 km. Using fibres with an outside diameter of100 μm and assuming a packing density of 78% (i.e. π/4), this wouldresult in a fibre-optic volume requirement of 900 cc.

[0024] This volume can be further reduced by a factor of four if 50 μmdiameter fibre was utilised. Further reductions in the fibre volumerequirement could also be achieved by the use of mirrored end fibres toproduce 2-pass ‘stub’ delay lines. The introduction of such 2-pass stubdelay lines could effectively halve the physical length of the fibretransmission lines 14.

[0025] If a range ambiguity such as those described above does exist,this may be addressed by using a known range finder in conjunction withthe apparatus according to the present invention. Alternatively, theapparatus may be adapted to eliminate range ambiguities. This might bedone by identifying which pixel transmits which signal to the ADP. Thiscould be done in a variety of different ways, including that describedbelow with respect to FIG. 3.

[0026]FIG. 3 shows an imaging laser-radar receiver similar to that ofFIG. 2, but only the fibre-optic transmission cables 14 from three ofthe pixels are shown for reasons of clarity. Each pixel has afibre-optic cable 14 for carrying light source information to a singleAPD 18. Each of the fibre-optic transmission cables 14 are of adifferent length as described with respect of FIG. 2, but differ fromthose shown in FIG. 2 in that they each have an alternative path 14 a.Thus light travelling along a fibre-optic transmission cable 14 arrivesat a junction, and can either travel along the original fibre-optictransmission cable 14 or can travel along the alternative path 14 a. Thealternative path is a fibre-optic transmission cable having the same orsimilar characteristics to the original fibre-optic transmission cable14. The alternative path 14 a joins up with the original fibre-optictransmission cable 14 prior to arrival at the APD18. The alternativepath 14 a is of a different length to the corresponding portion 14 b ofthe original fibre-optic transmission cable 14. Thus light which travelsalong the longer path (in this example 14 a, although the alternativepath could instead be adapted to be shorter than the original one)arrives at the APD 18 later than the light which travels along theshorter path (in this example 14 b), the difference being Δd. Thedifference in arrival times Δd of the two light pulses from any onepixel at the APD 18 is preferably very small in comparison to thedifference in arrival times of light pulses from the different pixels.(e.g. the difference between d1 and d8). Each pixel has an alternatepath of different length from those of other pixels, so that thedifference Δd is different for the two light pulses being carried fromeach of the pixels. This allows the processor unit 20 to identify whichpixel received the reflected light.

[0027]FIGS. 4a and 4 b help to explain how range ambiguities can beeliminated using the apparatus described with respect to FIG. 3. Therange is a function of the time taken for the light pulse to travel tothe body or target and back plus the time for the light pulse to travelalong the fibre-optic transmission cable 14.

[0028] The pulse train 50 is that which might be received by the APD 18when the apparatus of FIG. 2 is used. Time T_(o) is the time at whichthe first light source data 1 is received by a pixel. The delay in thesignals reaching the ADP is different for each pixel due to thedifference in cable length.

[0029] Assuming that light is received by the other pixels at virtuallythe same time, and that no range ambiguities exist, the pulse train 50would occur. Any range ambiguity would result in the pulses either beingin a different order than expected, or in fewer than a full set ofpulses (in this example nine) being received, and the processor will notknow which pixels received the light source data and therefore theprocessor will not know the cable length for those pixels and hence therange of the object.

[0030] If the apparatus of FIG. 3 is employed in the same situation,then the resulting pulse train 52 would occur. The difference betweensignals received from one pixel Δd is different for each pixel (Δd1,Δd2, etc.) and so each pixel receiving light source data can beidentified by the processor. Any range ambiguity resulting in fewer thana full set of pulses being received would not hamper range calculations,as the processor is able to identify the pixel receiving the lightsource data, and therefore the processor will know the cable length ofthe pixel and the range of the object can be calculated.

[0031] Instead of using different lengths of fibre-optic cable for thealternative paths 14 a, cables 14 a having different characteristicscould be used instead for each pixel.

[0032] It can be seen that the range ambiguity issue can be addressed ina variety of different ways.

[0033]FIG. 5 shows a second embodiment of the invention wherein adifferent arrangement of fibre-optic delay lines 30, 32 . . . isprovided. A pixel 24 hereinafter referred to as a ‘master’ pixel isconnected to an APD 18 via fibre-optic transmission line 28 in a similarmanner to that described in FIG. 2. The delay line 30 emanating from‘slave’ pixel f1 is connected into the delay line 28 of the master pixel24. Each of the remaining ‘slave’ f2, f3 . . . , f8 are similarlyconnected in series, with delay line 32 from pixel f2 being connectedinto the delay line 30 of fibre-optic f1, and onwards into optical fibretransmission line 28. The sequence of connecting the fibre-optic delayline from each associated pixel into its neighbour provides for a delayline structure where for example light signals 1 incident on pixel f8travel through fibre-optic delay line 44, on through fibre-optic delayline 42 associated with pixel f7, and similarly on through fibre-opticdelay line 40 associated with pixel f6 until the signal is finallytransmitted to the APD 18 via the master pixel optical fibretransmission line 28.

[0034] It will be evident that using the ‘in series’ delay linearrangement of FIG. 5 results in a time-shift effect for the slavepulses which are ultimately transmitted to the APD 18. Greaterconnection and interface losses will be expected with signals from thehigher order slave pixels, along with ghost pulses due to multiplereflections along the length of the delay line fibres.

[0035] When comparing the length of fibre required to effect a 24×24pixel array using the ‘in series’ arrangement, it follows that sixtyfour APD detector elements, each having eight×40 metres of fibre-optictransmission cable, results in an overall fibre length requirement of20.5 km. When based on a fibre outside diameter and a packing densityidentical to that used in the example given in the embodiment describedin FIG. 2, (i.e. 100 μm OD fibre and π/4 packing density) thiscorresponds to a fibre packing volume of 200 cc.

[0036] Alternative methods by which the delay may be introduced into theoptical transmission lines of any of the laser-radar receivers describedabove in accordance with the present invention also include, but are notlimited to, variations in the refractive index of the optical fibrematerial, and the use of optical fibres of differing materials.

[0037] Additionally, the cut ends 4 of the optical fibres 2 could becoated or covered with or by materials which act as filters, such as butnot limited to band pass, high pass or low bass filters, therebyproviding for the system to be designed to be responsive to particularranges of electromagnetic energy wavelengths.

[0038] The invention as described above could alternatively comprise afocal plane array assembly.

[0039] The examples as described herein relate to arrays of pixelelements formed into regular square elements (i.e. 3×3, 4×4, 5×5 etc.).This feature should not be construed as limiting the invention to arraypatterns of regular shapes. Arrays in accordance with the invention maycomprise pixels formed into regular or irregular or random layouts, bethey planar or non-planar (i.e. 2 or 3 dimensional). Arrays may also beconformal, in that their application is such that they are required tobe integrated into the outer surface such of a vehicle or body.

[0040] Additionally, the array size is not limited to one cluster ofpixels. Any number of clusters may be combined to produce an overallarray in accordance with the invention. Larger arrays may compriseclusters of pixels of any of the forms described by FIGS. 1, 2 or 3, orin any combination thereof.

[0041] Although short duration discrete pulses are preferable, it ispossible to receive pulses of longer duration, such as pulses which havea duration longer than the inter-pulse spacing. In this case, the APDwould experience a current which exhibited a step increase each time theAPD received an optical signal from a pixel.

[0042] The quality of information obtained from utilising the presentinvention can be tailored for individual applications, by adjustment tothe number of pulses within a given time period, and the duration ofthose pulses.

1. A laser-radar receiver comprising at least one array comprised of thefirst ends of a plurality of optical fibres, wherein the correspondingopposite ends of said optical fibres of the array are connected to thesame electromagnetic radiation detector means, each of said opticalfibres having differing physical characteristics which result in knownand differing delays in the transmission time of pulsed electromagneticradiation incident on said first ends of said optical fibres to theelectromagnetic detector means, wherein each of said optical fibres isadapted to present, along at least a part of its length, a choice of atleast a first path or a second path for the pulsed electromagneticradiation, the first path having different physical characteristics fromthe second path.
 2. A laser-radar receiver in accordance with claim 1wherein each of said first paths have different physical characteristicsfrom each other and wherein each of said second paths have differentphysical characteristics from each other.
 3. A laser-radar receiver inaccordance with claim 1 or claim 2 wherein said physical characteristicsinclude fibre length.
 4. A laser-radar receiver in accordance with claim1 or claim 2 wherein said physical characteristics include fibrerefractive index.
 5. A laser-radar receiver in accordance with claim 1or claim 2 wherein said physical characteristics include fibre material.6. A laser-radar receiver comprising an array comprised of the firstends of a plurality of optical fibres, wherein the correspondingopposite ends of said optical fibres are connected in series to thefirst ends of adjacent optical fibres, the final opposite end of thelast optical fibre in the series being connected to an electromagneticradiation detector means, this arrangement resulting in known anddiffering delays in the transmission time of pulsed electromagneticradiation incident on said first ends of the optical fibres andtraveling to the electromagnetic detector means.
 7. A laser-radarreceiver in accordance with any of claims 1 to 6 wherein said arrayforms a conformal array.
 8. A laser-radar receiver in accordance withany of claims 1 to 6 wherein said array forms a focal plane array.
 9. Alaser-radar receiver in accordance with any of claims 1 to 8 whereinsaid first ends of said optical fibres comprise means for selectivelyfiltering the wavelength of laser energy transmitted to the detectormeans.
 10. A laser-radar receiver in accordance with claims 1 to 9wherein said array comprises a plurality of smaller individual arrays.